1. Field of the Invention
The invention lies in the telecommunications field. More specifically, the invention relates to a method and a base station for data transmission in a wireless communications system, wherein a base station has an associated antenna device with a plurality of antenna elements, so that spatial resolution in a beam formation can be carried out.
In wireless (radio) communications systems, messages (for example voice, video information or other data) are transmitted with the aid of electromagnetic waves via a radio interface between a transmitting and a receiving station (base station and mobile, respectively). The electromagnetic waves are thereby transmitted at carrier frequencies which are in the frequency band laid out for the respective system. In the case of GSM (Global System for Mobile Communication), the carrier frequencies are in the region of 900, 1800 or 1900 MHz. For future mobile radio networks using CDMA or TD/CDMA transmission methods via the radio interface, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd generation systems, frequencies in the frequency band of about 2000 MHz are envisaged.
During their propagation in a propagation medium, signals are subject to interference from noise. As a result of diffraction and reflections, signal components flow over different propagation paths and are superimposed in the receiver, where they lead to mutual interference effects. Furthermore, if there is more than one signal source, the signals from these sources are superimposed. Frequency division multiplex (FDMA--frequency division multiple access), time division multiplex (TDMA--time division multiple access) or a method which is known as code division multiplex (CDMA--code division multiple access) are used to distinguish between the signal sources, and thus to evaluate the signals.
The GSM mobile radio system which is currently in existence is a wireless communications system using a TDMA component for subscriber separation (Time Division Multiple Access). Wanted information for the subscriber links is transmitted in time slots on the basis of a frame structure. Transmission takes place in blocks.
German patent application DE 195 49 148 discloses a mobile communications system which uses TDMA/CDMA subscriber separation (time division and code division multiple access) and uses a JD method (joint detection) at the receiving end in order to improve the detection of the transmitted wanted information on the basis of knowledge of the spread codes of a plurality of subscribers. Information from a plurality of wanted data links is transmitted simultaneously in one frequency channel (TCH traffic channel), and these links can be distinguished by their spread code.
German patent application DE 197 12 549 discloses the use of intelligent antennas (smart antennas) in order to increase the transmission capacity in the uplink (mobile to base station) direction. Various methods for space signal separation for the uplink (reverse--mobile to base station) and downlink (forward--base station to mobile) directions are known from Paulraj and Papadias, "Space-Time Processing For Wireless Communications," IEEE Signal Processing Magazine, November 1997, pages 49-83.
For the downlink direction, that is to say from the base station to the mobile, particular difficulties occur since the beam forming must be carried out before the transmitted signals have been influenced by the radio channel. Beam forming in the downlink direction is known from Schmalenberger and Blanz, "A Comparison Of Two Different Algorithms For Multi Antenna C/I Balancing," Proc. 2nd European Personal Mobile Communications Conference (EPMCC), Bonn, Germany, September 1997, pages 483-90, wherein a direct propagation path (line of sight) is assumed to exist between the radio stations involved, together with iterative calculation of beam forming vectors. Whenever the radio stations involved change, for example a link to or from a mobile station is set up or cleared, the entire calculation must be repeated.
2. Summary of the Invention
It is accordingly an object of the invention to provide an improved method and an improved base station for data transmission, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which the computation complexity for beam forming is considerably reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a signal transmission method in a wireless communications system of the type having a base station with an antenna device provided with and a plurality of antenna elements, and having a plurality of wireless stations, which method comprises:
defining space covariance matrices for a k.sup.th link from the base station to a wireless station and for a sum of further wireless stations;
defining a beam forming vector w.sup.(k) in accordance with the condition ##EQU1##
having a greatest intrinsic value .lambda..sub.max.sup.(k) in accordance with EQU R.sub.xx.sup.(k) w.sup.(k) =R.sub.I.sup.(k) w.sup.(k).lambda..sub.max.sup.(k)
for the link, where R.sub.xx and R.sub.I are the covariance matrices;
weighting transmission signals for the link with the beam forming vector; and
transmitting the transmission signals with the antenna elements.
In other words, the novel method for data transmission is used in a radio communications system with a base station and further radio stations. The further radio stations may be mobile stations, for example in a mobile wireless network, or fixed stations, for example in so-called subscriber access networks for wire-free subscriber access.
In a first step, space covariance matrices are defined for a k.sup.th link from the base station to a wireless station and for a sum of signals of further radio stations, that is to say, in the latter case, the sum of the interference for the link k.
In a second step, a beam forming vector w.sup.(k) is calculated for the link in such a manner that the relationship ##EQU2##
is maximized, wherein the generalized intrinsic vector w.sup.(k) is defined for the greatest generalized intrinsic value .lambda..sub.max.sup.(k) by using R.sub.xx.sup.(k) w.sup.(k) =R.sub.I.sup.(k) w.sup.(k).lambda..sub.max.sup.(k).
Transmission signals for the link are then weighted with the beam forming vector, and are supplied to the antenna elements for transmission.
This method is used to maximize the power available for the link k in comparison with the interference power, in which case it is possible to dispense with iterations, since the use of the secondary condition R.sub.xx.sup.(k) w.sup.(k) =R.sub.I.sup.(k) w.sup.(k).lambda..sub.max.sup.(k) for the calculation leads to the desired result in one step. The computation complexity is thus drastically reduced. Particularly in scenarios having a large number of subscribers and severely fluctuating channel conditions, beam forming in the downlink direction is considerably more economic according to the invention.
In accordance with an added feature of the invention, a transmission power is defined for the transmission signals for the link from the beam forming vector in accordance with P.sub.k =w.sup.(k)H w.sup.(k), where H is a complex-conjugate vector and P.sub.k is the transmission power, and raising the transmission power to a predetermined minimum signal-to-noise ratio in the radio station. The transmission power is also raised to a predetermined minimum signal-to-noise ratio in the wireless station. The instantaneous signal-to-noise ratio is measured in the receiving wireless station and is reported to the base station, so that the latter can use an appropriate transmission power setting to guarantee the minimum signal-to-noise ratio.
In accordance with an additional feature of the invention, data are transmitted in an uplink direction and a downlink direction in a given frequency band, and at least one of the space covariance matrices with respect to the link for the downlink direction is defined from measured values from the uplink direction. The correspondence between the transmission and reception frequency which exists using TDD (time division duplex) is thus used to reliably define the space channel parameters. There is thus no need to estimate the incidence directions for TDD systems.
If, on the other hand, data are transmitted in the uplink and downlink directions in mutually different frequency bands, then at least one of the space covariance matrices is defined for the downlink direction from dominant incidence directions for received signals with respect to the link in the uplink direction. Reliable space channel parameters can also be determined and continuously updated for this case.
In accordance with a further feature of the invention, a unit matrix is set for the space covariance matrix from a sum of influences from further wireless stations. This simplifies the calculation once again, with the generalized intrinsic value problem thus being reduced to a normal intrinsic value problem.
In accordance with again an added feature of the invention, the space covariance matrices are defined based on estimated channel pulse responses. In accordance with a further development, the channel pulse responses are derived from training sequences transmitted by the wireless station. This leads to particularly high reliability in the definition of the space covariance matrices. Here, the channel measurements are evaluated in addition to obtaining space statements. The channel impulse responses are advantageously defined from training sequences which are transmitted by the radio station. The transmitted training sequences for one or more links are known in the receiving station, so that particularly reliable estimated values can be determined.
In accordance with again another feature of the invention, the method further comprises, in the base station or a wireless station, jointly detecting a plurality of signals differing by virtue of a CDMA code, and eliminating interference from signals not associated with the respective link. Joint detection of a plurality of signals which differ by virtue of a CDMA code, and for interference from signals which are not associated with the link to be eliminated, is particularly advantageous. The requirements for the signal-to-noise ratio are thus reduced, and the transmission power levels required for adequate transmission quality may be reduced.
In accordance with again an additional feature of the invention, the space covariance matrix is set up for a sum of the influences of further wireless stations from received signals of signals not associated with the respective link and/or by joint detection of signals not being considered in the wireless station, in accordance with ##EQU3##
where M is a number of antenna elements in the antenna device and R.sub.I is the space covariance matrix.
With the above and other objects in view there is also provided, in accordance with the invention, a base station for data transmission in a wireless communications system with a plurality of wireless stations, comprising:
at least one transmitting device producing transmission signals;
an antenna device with a plurality of antenna elements connected to said transmitting device for transmitting transmission signals;
a control device connected to said transmitting device and a memory device connected to said control device, said control device being programmed:
to define space covariance matrices for a k.sup.th link from the base station to a wireless station and for a number of further radio stations, and to store the space covariance matrices in said memory device;
to define, for the respective link, a beam forming vector in accordance with the condition ##EQU4##
having a greatest intrinsic value .lambda..sub.max.sup.(k) in accordance with EQU R.sub.xx.sup.(k) w.sup.(k) =R.sub.I.sup.(k) w.sup.(k).lambda..sub.max.sup.(k)
and to store the beam forming vector in said memory device; and
to control a weighting of the transmission signals for the respective link with the beam forming vector.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and base station for data transmission in a radio communications system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.